"[The format is] very closely held," Eaton told Collider. "Steven [Moffat] crafts them, and Mark [Gatiss] writes some of them. It's a lot of work, and [Steven] also does Doctor Who and he worked on Tintin, so there couldn't be more than three."

Eaton argued that Sherlock co-creators Moffat and Gatiss use up all of their "creative juice" on the show.

"I think there will only ever be three at a time, if we're lucky," she said. "It's getting harder and harder to do another season, not just because Benedict and Martin [Freeman] are getting such high profiles, but Steven and Mark are busy and in demand."

Eaton also claimed that she was not surprised by the success of Sherlock in the US.

"I knew, as soon as I saw Sherlock, that it was going to be special," she explained. "The fact that it caught fire the way that it did, didn't surprise me."